This invention relates to fuses for elevated circuit voltages, e.g. 15 kv, wherein the fusible element means are wound helically to make it possible to arrange within a casing, or fuse tube, of a given length fusible element means which exceed that length. It is very often desirable to arrange between the terminal elements of a fuse a plurality of parallel-connected fusible element means, e.g. in form of silver ribbons having points of reduced cross-sectional area spaced from each other in a direction longitudinally of the ribbons. The high-current interrupting performance as well as the low-current interrupting performance of parallel connected fusible elements in ribbon form is superior to that of a single fusible element of equal current-carrying capacity. Generally a plurality of parallel connected fusible elements is wound around a common support, or mandrel, of insulating material. In this instance the individual helically wound fusible elements are arranged in coaxial relation, have the same pitch, and have a predetermined fixed spacing from each other. When the number of helically wound parallel connected fusible elements is large, the axial spacing between parallel connected fusible elements is small, and this adversely affects the operation of the fuse. In particular, the presence of a large number of helically wound axially narrowly spaced fusible elements tends to result, when the fuse blows, in a merger of parallel current paths into one single current path. In other words, and being more specific, there occurs a breakdown of the insulation between axially spaced points of the parallel current paths by excessive growth and consequent merger of the fulgurites formed at such points. A reduction of the number of helically wound parallel connected fusible elements results in an increase of the axial spacing between the turns of the fusible element means, but may result in intolerable conditions attributable to excessive metal concentration and reduction of the interface between the fusible element means and the arc-quenching filler. Reduction of the number of parallel-connected fusible elements may also be detrimental to the low overload current performance of the fuse.
Some of the above limitations may be avoided by resorting to a plurality of parallel connected helically wound fusible elements which are not arranged coaxially but more or less evenly distributed over the cross-sectional area of the fuse tube. A typical member of this family of fuses is disclosed in U.S. Pat. No. 3,571,775 to F. J. Kozacka et al, Mar. 23, 1971, for HIGH-VOLTAGE FUSE HAVING A PLURALITY OF HELICALLY WOUND RIBBON FUSE LINKS. Fuses as disclosed in that patent are limited to helical fusible elements of critically small diameters -- from three-sixteenths to one-half inch -- which critically limits the length of the fusible elements that can be arranged inside a fuse tube of given length.
In another kind of fuses such as, for instance, those disclosed in U.S. Pat. No. 3,460,085 to B. T. McAllister et al, Aug. 5, 1969, for FUSE AND FUSE ELEMENT SUPPORTS FOR USE THEREIN a pair of helically wound fusible element means is arranged side by side in a common tubular casing or fuse tube. One limitation of this structure consists in that its fusible element means require relatively complex positioning means, and more importantly that it lends itself not to a subdivision of each of its two helically wound fusible element means into a plurality of axially spaced parallel connected sub-elements of equal pitch.
Most prior art high-voltage fuses lacking a support for a plurality of helically wound fusible elements are subject to the danger of distortion of their fusible elements during the process step of filling a pulverulent arc-quenching filler into the casing or fuse tube.
Fuses manufactured and constructed in accordance with U.S. Pat. No. 3,848,214, 11/12/74, to Erwin Salzer, for METHOD OF ASSEMBLING ELECTRIC HIGH-VOLTAGE FUSES AND SUBASSEMBLY THEREFOR are not subject to this limitation, but the temporary fusible element supporting structure disclosed in that patent is only applicable to a coaxial arrangement of all parallel-connected helically wound fusible elements. This, in turn, precludes to maximize the degree of dispersion of fuse link metal in the arc-quenching filler material or, in other words, to maximize the interface between fuse link metal and arc-quenching filler material.
It is the prime object of the present invention to provide electric fuses for elevated circuit voltages that do not require a permanent insulating mandrel or the like for supporting the helically wound fusible elements thereof, whose fusible elements are not subject to the danger of displacement of distortion during the process step of filling the casing of the fuse with a pulverulent arc-quenching filler, and whose helically wound fusible elements are arranged around parallel, spaced axes rather than in coaxial relation.
Another object of this invention is to provide a process for manufacturing electric fuses having the above referred-to desirable features.